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Arthropoda, as a phylum, includes the crustaceans, arachnids and insects. This is the largest phylum in terms of the number of species, as it includes perhaps three quarters of the million or so described species. These groups probably had their origin in the Cambrian period or before, and divergence began early—thus accounting for such a numerous and diverse phylum. These are segmented animals, much like the annelid worms, but the arthropods have evolved a harder outer shell, or cuticle, which serves as an external skeleton. Some of the larger crustaceans have taken this hard outer shell to an extreme—the crabs, shrimps and lobsters. But, this organic armour comes at a price. It is rigid, and must be shed for the animal to grow and mature during a process called apolysis (separation of the shell from the epidermis), followed by ecdysis (shedding of the old shell). During this process, the outer “cornea” of the eye will be shed and replaced with a new one. The cover images show a living lobster’s eye and …
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